Preproenkephalin messenger RNA-expressing neurons in the rat parabrachial nucleus: subnuclear organization and projections to the intralaminar thalamus.

The pontine parabrachial nucleus, which is a key structure in the central processing of autonomic, nociceptive and gustatory information, is rich in a variety of neuropeptides. In this study we have analysed the distribution of parabrachial neurons that express preproenkephalin messenger RNA, which encodes for the precursor protein for enkephalin opioids. Using an in situ hybridization method, we found that preproenkephalin messenger RNA-expressing neurons were present in large numbers in four major areas of the parabrachial nucleus: the Kölliker-Fuse nucleus, the external lateral subnucleus, the ventral lateral subnucleus, and in and near the internal lateral subnucleus. Many preproenkephalin messenger RNA-expressing neurons were also seen in the central lateral subnucleus, and in the medial and external medial subnuclei. Few labeled neurons were found in the dorsal and superior lateral subnuclei. Injection of the retrograde tracer substance cholera toxin subunit B into the midline and intralaminar thalamus demonstrated that the enkephalinergic neurons in and near the internal lateral subnucleus were thalamic-projecting neurons. Taken together with the results of previous tract-tracing studies, the present findings show that many of the enkephalinergic cell groups in the parabrachial nucleus are located within the terminal zones of the ascending projections that originate from nociresponsive neurons in the medullary dorsal horn and spinal cord, as well as from viscerosensory neurons within the nucleus of the solitary tract. The enkephalinergic neurons in the parabrachial nucleus may thus transmit noci- and visceroceptive-related information to their efferent targets. On the basis of the present and previous observations, we conclude that these targets include the intralaminar and midline thalamus, the ventrolateral medulla and the spinal cord. Through these connections, nociceptive and visceroceptive stimuli may influence several functions, such as arousal, respiration and antinociception.